Electric wave amplifying system



Feb. 27, 1940. J. o. SMETHURST' F ELECTRIC WAVE AMPLIFYING SYSTEM FiledMarch so, 1957 I ll ll -IIIF INVENTOR' J.0.$ME THURS T ATTORNEK PatentedFeb. 27, 1940 ELECTRIC WAVE AMPLIFYING SYSTEM Joseph 0. Smethurst,Bloomfield, N. 3., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationMarch 30, 1937, Serial No. 133,814

7 Glaims.

This invention relates to systems involving electric space dischargedevices, as for example electric wave amplifying systems.

An object of the invention is control of transmission properties of suchdevices or systems, as for example,-their gain or transmissionefficiency, load carrying capacity, and modulation or distortion. 1

Inone specific aspect the invention is an amplifier with anode and gridcircuits comprising a common impedance having two parallel branches,

one branch having its direct current impedance high compared to theother and having a variable resistance for providing variable negativefeedback of the waves to be amplified and the other branch havingresistance traversed by the anode direct current for providing gridbiasing voltage and having inductance of high reactance for the wavestobe amplified. For instance, the one branch may have, in'serieswith-the variable resistance therein, a condenser of low reactance for"the Waves to be amplified, for preventing passage of direct currentthrough that branch; and the other branch may have, in series with theresistor for providing grid biasing voltage, an inductance coil forsubstantially preventing passage 'of the vaves to be amplified throughthat branch. By varying the resistance in the one branch, the amount ofthe negative or gain-reducing feedback can be varied to vary or adjustthe amplifier gain, for example through a range of several times tendecibels, without causing the grid biasing voltage to vary, andtherefore without deleteriously affecting the power output capacity ofthe amplifier. 1

Other objects and features of the invention will be apparent from thefollowing description and claims. I The single figure of the drawingshows an amplifier embodying the specific form of the invention referredto above.

The amplifier comprises apentode tube 1 for amplifying speech or othersignal waves supplied to tube I from incoming lineor circuit 3 throughinput transformer 4 and transmits the amplified waves through outputtransformer 6 to outgoing cathode, and then through resistance 2| andchoke coil 22 to the negative pole of the battery. Screen grid potentialfor the tube is supplied from the battery through. choke coil 25'whichmay have the same magnetic core 26 as coil 22.

Condensers 23 and 29 are lay-pass condensers of low reactance foralternating current.

The voltage across resistance 2! and choke coil 22 due to flow of plateand screen direct currents therethrough supplies grid biasing potentialfor I the control grid of the tube.

The alternating current output circuit of the tube extends from thecathode through condenser resistance 32, condenser 28, andbranchcircuits M and I5, to the plate. The voltage across condenser 3iand resistance 32 due to fiow of alternating current therethrcughproduces nega-' tive or gain-reducing feedback in the tube.

Negative feedback is advantageous for example for reducing themodulation and noise in the amplifier and increasing its gain stability,as

grid biasing voltage to vary, and therefore without deleteriouslyaffecting the power output capacity of the amplifier or reducing theundistorted power that the amplifier iscapable of delivering to theload. l

For signaling frequencies or alternating current, the reactance ofcondenser 38 may be'negligibly low and the admittance of coil 22 may benegligibly low. The ratio of the impedance of the path throughresistance 2! and coil 22-to the impedance of the path through condenser3! and 45 resistance may be low for direct current and high for signalfrequencies or alternating cur.- rent. Y

For signal frequencies or alternating current, the impedance of coil 25may be sufiiciently high to substantially prevent shunting of currentfrom the path through condenser 31 and resistance 32 by thepath throughcondenser 29 and coil 25.

By way of example, with the tube I a Western Electric Companys 310A or311A type of tube: I 55 condenser 31 may have a capacity of 4microfarads; condenser 28 a capacity of 4 microfarads; condenser 29 acapacity of 2 microfarads; resistance 32 in its maximum setting a valueof 3,140 ohms when its desired range of gain variation is for instance20 decibels, or a value of approximately 100,000 ohms when its desiredrange of gain is for instance approximately decibels; resistance 2I avalue of 150 ohms; coil 22 a direct current resistance of 350 ohms; andcoil 25 a direct current resistance of 350 ohms. Coils 22 and 25 may bewound on core 26 as shown, so that current flowing to the left throughcoil 22 acts cumulatively with current flowing to the left through coil25.

With the 311A tube, resistance I2 may be shortcircuited and a portion ofthe winding I5 omitted from circuit, as indicated in the drawing. Whenthe 310A tube is used, the entire resistance I 2 and the entire windingI5 may be included in circuit. By way of example, when the circuit is tobe used with these two types of tubes, the resistance l2 and I3 may havevalues of 86,500 ohms and 13,500 ohms, respectively.

What is claimed is:

1. A system for translating signaling waves comprising an electric spacedischarge device having anode and grid circuits and gain control meansfor said device comprising an impedance common to said circuits havingtwo electrically separate branches in parallel relation, one of saidbranches comprising in series a capacity and a resistance traversed bysaid signaling waves for providing negative feedback of the waves to betranslated and the other of said branches comprising inductance andeffectively in series therewith resistance traversed by anode directcurrent for providing grid biasing voltage, and means for varying saidfirst-mentioned resistance sumcient- 1y to vary said negative feedbackthrough a substantial range.

2. A system for translating signaling waves comprising an electric spacedischarge device having anode and grid circuits and gain control meansfor said device comprising a two-terminal network common to saidcircuits, said network having two branches electrically separatedthroughout their lengths, one having its direct current impedance highcompared to the other, a condenser in said one branch and in seriestherewith a resistance in said one branch traversed by said signalingwaves for producing gainreducing feedback of the waves to be translated,means for varying said resistance sufiiciently to produce substantialgain variation of said device for said waves to be translated, and insaid other branch a resistance traversed by the anode direct current forproviding grid biasing voltage and an inductance effectively in seriestherewith for substantially preventing passage therethrough of the meansfor varying said resistance sufiiciently to produce gain variation ofsaid device for the waves to be translated at least of the order of tendecibels, over the frequency range of said waves, and direct currentcathode-anode and cathodegrid' circuits for said device having a commonimpedance comprising effectively in series an inductance of highimpedance to waves to be translated and a resistance traversed by anodedirect current for providing grid biasing voltage for said device, saidimpedances being electrically separated except at their ends.

4. A wave translating system comprising an electric space dischargedevice having anode, cathode, control grid and screen grid electrodes,alternating current cathode-anode and cathodecontrol grid circuits forsaid device having a common impedance comprising a condenser and inseries therewith a variable resistance for producing variable negativefeedback in said device, direct current cathode-anode andcathode-control grid circuits for said device having a common impedancecomprising a choke coil with a magnetic core, said impedances beingelectrically separated except at their ends, a direct currentcathode-screen grid circuit for said device comprising a choke coilhaving a winding on said core, and a by-pass condenser connecting saidcathode to a point on said last-mentioned circuit between saidlast-mentioned winding and said screen grid.

5. In a Wave translating system, a screen gridv tube, a space currentsupply source therefor, a path including an inductance coil connectedbetween the tube cathode and the negative pole of said source, a circuitincluding said coil con-' necting the cathode and the control grid ofthe tube, and a connection including a second inductance coil betweenthe screen grid and a point of positive potential on said source, saidcoils being coupled in series opposing'inductiv'e relaf tion.

6. A wave translating system comprising an electric space dischargedevice having two grids, a circuit comprising an inductance coilconnecting the cathode to one of the grids, a second circuit comprisinga second inductance coil and said first inductance coil in serialrelation con necting the cathode to another of the grids, said coilsbeing so inductively related that for current flowing in series throughsaid coils the flux which is produced by the current in one coil isdecreased by the current flowing in the other coil, and a capacity oflow reactance for frequencies of the waves to be translated connectedacross a portion of said second circuit including said coils.

7. A wave translating system comprising an electric space dischargedevice having an anode, a cathode, a discharge control grid, a secondgrid, a circuit comprising a source of direct cur rent connecting saidcathode and said anode, a circuit connecting said cathode and saidcontrol grid, an inductance coil commonto said circuits, a pathconnecting said cathode and said second grid and including said coilsand said source in serial relation, with said source connected be-JOSEPH O. SMETHURST.

